spice: load module when enabled on the cmdline

In case the spice opts are not registered, try loading the spice module.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20201019075224.14803-10-kraxel@redhat.com